lloret has much more nightlife than tossa, and is really geared up for those who want a beach/beer holiday. tossa seems almost sedate by comparison, but does have a good beach and some lovely restaurants.
it really depends on what you want from your holiday.
a few websites to look at -i assume you have checked out
http://www.ferriscoachholidays.co.uk/ and http://www.siestaholidays.co.uk/

are
http://www.lloretdemar.org/
http://www.lloret.net/index_e.php
http://www.infotossa.com/indexeng.php
and not forgetting
http://www.waterworld.es/index.php?lang=uk

I was in Tossa last year and thought it was lovely. Enough there to keep you going even though we were early season. My friends went to Lloret and didn't like it at all but as jim says it depends what your looking for?

As Jim said it depends on what you want ..........Tossa is more laid back than Lloret which is livelier and more geared up for nightlife. I would say Lloret is more central to get to other resorts for a night out ...... ( Malgrat and Calella )